1. Field of the Invention
The present invention relates to a rotary electric machine, in which a surface of commutator is rubbed by a shakedown brush in advance, and a method of producing the rotary electric machine, in particular to a rotary electric machine suitable for an electromotive power steering motor, and a method of producing the rotary electric machine.
2. Discussion of Background
FIG. 2 is a cross-sectional view of a part of conventional rotary electric machine for showing a structure and sliding traces of brush of the conventional rotary electric machine. In FIG. 2, numerical reference 1 designates a yoke of the rotary electric machine; numerical reference 2 designates four magnets located on an inner peripheral surface of the yoke 1 at even intervals; numerical reference 3 designates an armature of the rotary electric machine having a shaft 4 and a commutator 5, wherein a predetermined gap is provided between the inner peripheral surface of magnets 2; and numerical reference 6 designates a bracket covering an opening end of the yoke 1, wherein the shaft 4 is supported by a bearing 7 located in the yoke 1 and a bearing 8 located in the bracket 6 so as to be rotatable.
Numerical reference 9 designates an attachment of brush holder, located in the bracket 6; and numerical reference 10 designates a brush holder located in the attachment 9 of brush holder having, for example, four holding portions in a circumferential direction, and holding brushes 11 and springs 12 inside thereof. For example, the number of the brushes 11 is four, each of the brushes rubs against the surface of the commutator 5. The number of the springs 12 is four, and the springs apply a predetermined pressure force respectively between the brushes 11 and the commutator 5. The brushes 11 are different from a shakedown brush, to be described below, and ordinarily assembled in this kind of rotary electric machines.
Numerical reference 13 designates sliding traces produced by the rubbing motions of the brushes 11 against the surface of the commutator 5, wherein the sliding traces are schematically shown to indicate an area of the sliding traces. Because the brushes 11 are shaped as in FIG. 2, two sliding traces 13 are produced. As the brushes 11 wear out, the two sliding traces are gradually widened, and finally the entire sliding surfaces of the brushes rub against the commutator. The brushes 11 are shaped so that the both ends rub against the commutator 5 in order to progress aging between the brushes and the commutator 5 as early as possible in a primary stage after completing to assemble the rotary electric machine, whereby noise is early reduced. This shape of the brushes 11 is adopted in conventional techniques.
In the rotary electric machine constructed as described above, when the armature 3 rotates, the commutator 5, fixed by pressing into the shaft 4, rotates in the same direction at the same rotational speed as that of the armature 3. When the commutator 5 rotates, the brushes 11 slide on the surface of the commutator 5 and are pressed and in contact with the commutator 5 by the springs 12. In particular, immediately after completing to assemble the rotary electric machine, a sliding condition between the surface of the commutator 5 and the brushes 11 is not stabilized. Therefore, in order to reduce noise caused along with the sliding motion by the brushes 11 and stabilize the performance of the rotary electric machine, a relatively small current is applied to the rotary electric machine after completing to assemble the rotary electric machine, and the armature 3 is rotated to stabilize the sliding motion between the brushes 11 and the surface of the commutator 5. This is generally called “shakedown”, wherein because the shakedown process requires a substantial time, a drop of productivity and a cost increment are caused.
Further, because a carbon coat is not produced on the surface of the commutator 5 immediately after completing to assemble the rotational electric machine, a relatively large current applied to the rotary electric machine for measuring the performance is apt to generate spark. Therefore, the spark roughens the surface of the commutator 5, whereby noise is caused and lifetimes of the commutator and the brushes are shortened. To deal therewith, it is necessary to measure the performance after the shakedown process, whereby the drop of productivity and the cost increment are caused.
Further, when the rotary electric machine is applied in particular to a motor for an electromotive power steering device, because manipulation of a steering handle variously changes in accordance with driving conditions, a rotational speed and a rotational direction of the motor, interlocked with the steering handle, variously change, and noise easily bothers a driver. Further, along with a tendency of designing silent cars, it is required to obtain strict silence of the motor for electromotive power steering devices. Therefore, the shakedown process is performed for a relatively long time, whereby the drop of productivity and the cost increment are extensively caused in particular in mass-produced rotary electric machines.